On the occasion of employing a digital modulation system etc in wireless communications, an important technology is a technology of restraining a non-linear distortion by linearizing an increase characteristic of a power amplifier. The power amplifier, which restrains the non-linear distortion, is exemplified by a distortion compensating amplifier using a distortion compensation coefficient.
FIG. 1 is a diagram representing an example of a configuration of a conventional distortion compensating amplifier (Amplifier).
The distortion compensating amplifier includes a multiplier which multiplies a baseband signal X(I, Q) by a LUT (Look Up Table) parameter h(|X|), a digital-to-analog converter (DAC: Digital-Analog Converter), a quadrature converter (QMOD: Quadrature Modulator), a gain amplifier, a directional coupler (DC), an antenna, a down-converter (MIX: Mixer), an analog-to-digital converter (ADC: Analog-Digital Converter), a demodulator (DEM: Demodulator), a subtractor, an amplitude calculating unit (|X|), and a LUT unit.
When the baseband signal X(I, Q) is inputted to the distortion compensating amplifier, X(I, Q) is multiplied by the LUT parameter h(|X|) which undergoes addressing with an amplitude (|X|) of the baseband signal. A multiplied result is converted by the DAC into analog data. The QMOD quadrature-modulates the analog data which has thus been analog-converted. The quadrature-modulated data is, after a gain has been acquired by the amplifier, transmitted as an RF (Radio Frequency) output Y from the antenna.
Further, the directional coupler monitors the transmission output Y. The ADC in the frequency converter (MIX, down-converter), after down-converting the output Y into a sampling-enabled frequency, converts the output Y into the digital signal. The thus-converted digital signal is demodulated as baseband data Y(I, Q) by the DEM. The distortion compensating control is done so that the output signal of the amplifier becomes linear (linear type) with respect to the input signal by updating the distortion compensation coefficient in the LUT so as to make the demodulation data Y (I, Q) coincident with the baseband signal X(I, Q) on the reference side.
FIG. 3 is a diagram representing a principle of the distortion compensation.
An error component is obtained by comparing an input |X| with an output |Y|, and the distortion compensation parameter h(|X|) in the LUT is updated so that the error component gets approximate to “0”. Herein, for simplicity, only correction of the amplitude will be explained. As a matter of fact, a phase rotation is also corrected.
FIG. 3 represents a characteristic curve (with no compensation of the distortion) of the output |Y| with respect to the input |X|. In an idealistic amplifier, the output is proportional to the input. Generally in the amplifier, however, the gain reduces when getting close to a saturation point, and hence the output Y decreases. The distortion compensation is defined as an operation of making the correction so that the output becomes linear (linear type) by multiplying the gain for the decrease in the output by the input |X| beforehand.
A general expression is given by the formula (1).Y=X·h(|X|)·G  Formula (1)
Herein, G is the gain of the amplifier. Further, h(|X|) is the LUT parameter read from the LUT. The correction is made so that the output becomes linear (linear type) with the LUT parameter h(|X|).
The linearity characteristic of the output Y can be improved by updating the LUT parameter (distortion compensation coefficient) h(|X|) so that the error component between the input |X| and the output |Y| becomes “0”.
FIG. 4 is a diagram illustrating a configuration of the LUT unit. A new distortion compensation coefficient h′(|X|) in such a direction as to compress the error component is calculated from an error component e(I, Q) obtained from the input |X| and the output |Y| and from the distortion compensation coefficient h(|X|) used last time and is written to the address |X| in the LUT memory of the LUT. A coefficient updating method of updating the distortion compensation coefficient h(|X|) by the coefficient updating unit is described in the Patent document 4.
If the distortion compensation coefficient is converged, the formula (2) is established.X=X·h(|X|)·G·Gfb  Formula (2)
Hence, h(|X|) is expressed in the following formula (3).
                                          h            ⁡                          (                                              X                                            )                                ⁢          ••          ⁢                      1                          G              ·              Gfb                                      ⁢                                                      Formula        ⁢                                  ⁢                  (          3          )                    
Herein, Gfb is a feedback gain (FB gain: Feedback Gain).
FIG. 2 is a diagram representing an example of the distortion compensating amplifier having a gain adjusting function unit. Components other than the gain adjusting function unit are substantially the same as those in the example of the distortion compensating amplifier in FIG. 1.
The transmission output can be adjusted with a scheme that the FB gain changes in a way that provides an FB gain adjusting function.
When adjusting a gain adjustment volume, a braking voltage of VATT (Variable Attenuator) fluctuates. When a quantity of attenuation of the VATT fluctuates with the result that the FB gain changes. When the FB gain changes, the distortion compensation coefficient is updated, and an output level changes.
In the case of the configuration in FIG. 2, the distortion compensation coefficient is used for changing the output level. The FB gain is multiplied by k for adjusting the output level.
At this time, when the new distortion compensation coefficient is converged, the following formula (4) is established.X=X·h′(|X|)·G·Gfb·k  Formula (4)
Therefore, h′(|X|) is given in the following formula (5).
                                                        h              ′                        ⁡                          (                                              X                                            )                                ⁢          ••          ⁢                      1                          G              ·              Gfb              ·              k                                      ⁢                                                      Formula        ⁢                                  ⁢                  (          5          )                    
From the formulae (3) and (5), a relationship between the original distortion compensation coefficient h(|X|) and the new distortion compensation coefficient h′(|X|) becomes as represented in the following formula (6).
                                                        h              ′                        ⁡                          (                                              X                                            )                                ⁢          ••          ⁢                                    h              ⁡                              (                                                    X                                                  )                                      k                          ⁢                                                      Formula        ⁢                                  ⁢                  (          6          )                    
Accordingly, when the FB gain is multiplied by k, the distortion compensation coefficient (LUT coefficient) turns out to be 1/k-fold. As a result, an output Y′ after multiplying the FB gain by k becomes as given in the following formula (7).Y′=X·h(|X|)·(1/k)·G=Y·(1/k)  Formula (7)
As a result of multiplying the FB gain by k, the output Y′ becomes 1/k-fold of the original output Y.
FIGS. 5 and 6 are diagrams each representing an example of a range of the distortion compensation coefficient.
With respect to the distortion compensation coefficient, the real part thereof is set in a range of 0 through 2, and the imaginary part thereof is set in a range of −1 through +1. If a value of the distortion compensation coefficient rises due to the adjustment of the FB gain, the range usable for the linearity correction of the distortion compensation reduces.
For example, as in FIG. 5, the distortion compensation coefficient adjusted to “1” when shipped from the factory is, if adjusted to a through the adjustment of the FB gain, capable of only 2/α-fold compensation though capable of 2-fold compensation when shipped from the factory.
Further, according to the method of changing the output level by adjusting the FB gain, there is no change in the output level immediately after adjusting the FB gain, and the output level does not change till the distortion compensation coefficient (LUT coefficient) is updated with the change in the FB gain. Accordingly, a time lag occurs till the output level changes since the FB gain has been adjusted. Actually, there might be a case in which the time lag on the order of 1 sec occurs.    [Patent document 1] Japanese Patent Laid-Open Publication No. H05-129845    [Patent document 2] Japanese Patent Laid-Open Publication No. 2005-5834    [Patent document 3] Japanese Patent Laid-Open Publication No. 2002-176321    [Patent document 4] International Publication WO 01/008320